hi,

its just add this code to functions.php?

add_filter('avf_logo','av_change_logo');
function av_change_logo($logo)
{
    if( is_page(21) ) {
		$logo = "http://www.domain.com/wp-content/uploads/logoforpage21.jpg";  
	} elseif ( is_page(22) ) {
		$logo = "http://www.domain.com/wp-content/uploads/logoforpage22.jpg";  
	} elseif ( is_page(23) )  {
		$logo = "http://www.domain.com/wp-content/uploads/logoforpage23.jpg";  
	}

    return $logo;
}